1. Field of the Invention
The invention relates to the field of leak prevention from a tank having a liquid such as oil therein. It is primarily designed for use in the cargo tanks of ships or any other water borne vessel and, in particular, to a self-contained leak-prevention system for such vessels.
2. Description of Related Art
The disastrous results that can occur when an oil tanker runs aground or collides with another ship are well known, one has to look no further than the Exxon Valdez, which ran aground on Bligh reef in Prince William Sound near Valdez, Ak. spilling 11,000,000 gallons of crude oil. To date, the oil has not been completely removed from the Alaska shoreline. In fact, it is estimated that less then 10 percent of the oil has been recovered. Thus, there has and will continue to be a need for on-board systems to prevent such catastrophes from occurring.
Many solutions have been proposed to reduce the possibility of a major spill from a tanker, with one of the most notable being the recently passed legislation to require all tankers to have double hulls. While the use of dual hulls may reduce the chance of a major oil spill, it will not entirely eliminate such an event from occurring. The momentum of a tanker, when underway, is so enormous that the force produced upon impact with another ship or upon running aground breach both hulls. The use of dual hulls also reduces the cargo carrying volume of the tanker and significantly raises the transportation costs. Additionally, the cost of retrofitting double hulls into existing tankers is extremely expensive, about 15% to 25% of a similar new tanker.
Another approach is to attach a curtain about the periphery of the tanker at a position slightly below deck level. The curtain is stored in a "folded" condition and, when a tank is punctured, the curtain is dropped into the water. The ends of the curtain are weighted causing the curtain to sink to a depth below the keel of the ship. Thus, theoretically, the oil, being lighter than water, would be trapped between the curtain and the hull of the ship. A somewhat similar approach is the use of an inflatable tubular barrier that can be extended about the ship at a short distance therefrom. But these solutions may not always be effective. For example, in high seas or heavy currents, the curtain or inflatable barrier could be ripped apart. If the ship runs aground, the curtain or barrier may not be able to extend sufficiently to be effective.
In general, and particularly in high seas, it is better to prevent the oil from leaking out of the ship rather than to contain the oil locally. Thus, most oil spill prevention systems attempt to prevent or limit the oil from ever leaving the ship in the first place. This is what the previously mentioned double-hull tanker design is attempting to accomplish. An additional idea is to divide the storage tanks into many small compartments by means of bulkheads connected by normally opened valves. Thus, should the hull be punctured, the valves controlling flow to the breached compartments can be closed, limiting the spill to a "relatively" small amount of oil. However, such a system will significantly increase the weight and cost of the tanker. It is also doubtful that such a system could be easily retrofitted to the existing tanker fleet.
Another approach is to "pull" a vacuum from within the tanks to essentially equalize the pressure of the oil and surrounding water. This concept has a major disadvantage because, unless the tanks have a perfect seal, the vacuum pumps must operate continuously during routine tanker operations. In addition, equalizing pressures only eliminates the initial force causing the oil to be expelled from the tank. There are always secondary leak mechanisms, such as currents and high seas. Additionally, there is the phenomenon created by the different surface properties of oil and water, which cause the oil to leak out of the tank at high rates. Tankers, as well as barges, have lost their entire cargo, proving that this phenomenon plays a significant role in causing oil to leak out of a tank. Using the same theory, the oil above the water line could be transferred to other tanks, thus, also equalizing the forces between the oil within the tank and the surrounding water. However, there are three major disadvantages with this concept. First, if plumbing is designed to allow oil to be transferred to the ballast tanks, then the likelihood of oil spills caused by the inadvertent transfer of oil to the cargo tanks is increased. Secondly, the tanks are loaded quite carefully in order to avoid inducing extreme loads on the ship during transit. The loads caused by relocating the oil on an already damaged ship could cause even more damage. Thirdly, very high-pressure and high-flow-rate pumps will have to be retrofitted for the concept to be effective. Another approach is to fill the tanks only to the waterline; however, this procedure will severely impact the cost of transporting the oil. For example, it has been estimated by the National Transportation Board that, if the Exxon Valdez used this approach, it would carry some three million dollars less oil per voyage.
To avoid the loss of oil carrying tank volume, several systems have been developed that allow the use of the cargo tanks for carrying the seawater ballast. One of these is disclosed in U.S. Pat. No. 3,707,937 "Anti-Pollution Ballast Container" by H. Likes. Likes discloses a bellows type tank mounted in the bottom of one or more of the fuel tanks. Seawater can be pumped into the bellows causing it to expand upward into the tank. Thus, the ballast can be isolated from the oil contaminated tank and can be latter dumped without fear of contaminating the surrounding area. A similar "collapsible tank within a tank" system has been developed by Diatank Ltd. Ontario, Canada. In this system, the collapsible tank comprises a moveable top lid attached to the side of the tank by a flexible fabric sheet. Seawater can be pumped from the exterior of the ship into the collapsible lower tank. Thus, the seawater is separated from the "empty" contaminated portion of the oil tank. In another version, the flexible fabric is attached to the top of the oil tank, thus, when carrying oil, the flexible fabric serves as a liner for the side and a portion of the bottom of the oil tank.
An additional approach is to line the interior of the tank with a high-strength, flexible fabric made from materials such as DuPont Corporation's KEVLAR brand aramid fibers. In reality, the interior of the tank is far from smooth and free from internally mounted structures such as ladders, pipes, tank-washing systems, heating coils and pumps. It is doubtful that the liner could be continuous and would have many joined sections and openings that would have to be sealed to internal tank structure. Therefore, a rigorous inspection program would be required to insure that the liner is intact. In addition, the liner would interfere with routine procedures, such as tank maintenance and inspection. The liner would have to be removed in order to allow inspection of major tank integral structure. The liner could also become impregnated with volatile compounds and flammable gases can become trapped behind the liner.
A system of interest can be found in U.S. Pat. No. 3,906,880 "Oil Recovery Apparatus For A Tanker" by L. Hebert. Hebert discloses an apparatus for preventing significant amounts of oil from draining from a breached tank. In detail, the apparatus comprises a flexible container attached to the top of the four walls of the tank and covering the top thereof. A flexible wire mesh net shield is positioned underneath the container and attached to the side walls of the tank. Upon a breach of the tank, the wire mesh net can be dropped down so that it lines the interior surface of the outer wall of the tank to protect the container from possible damage that could be caused by the jagged edges of the hole. Pumps are used to transfer the oil from below the container to above it as the container lowers down into the tank, lining the walls and bottom of it.
While the above system might work, it has several disadvantages. First, the container, when in the stored position, covers the entire top surface of the tank, making it difficult to access to the interior of the tank. Additionally, as previously mentioned, the interior of the typical tank contains a significant amount of structure, including ladders, pipes and pumps. Thus, the lowering of the mesh net and container will be difficult to accomplish.
It is therefore a primary object of the subject invention to provide an apparatus that can prevent or greatly reduce the spillage of oil from the cargo tank of a ship when the tank is breached.
It is another primary object of the subject invention to provide an apparatus that can be easily incorporated into existing ships and can prevent or greatly reduce the spillage of oil from the cargo tank of a ship when the tank is breached.
It is another object of the subject invention to provide an apparatus that can be easily incorporated into existing ships without creating the need for significantly altering procedures and that can prevent or greatly reduce the spillage of oil from the cargo tank of a ship when the tank is breached.
It is still another object of the subject invention to provide an apparatus that can prevent or greatly reduce the spillage of oil from the cargo tank of a ship when the tank is breached that can be stored in a minimal space within the tank.
It is a further object of the subject invention to provide an apparatus that can prevent or greatly reduce the spillage of oil from the cargo tank of a ship when the tank is breached that does not require continuous power after being fully activated.
It is a still further object of the subject invention to provide an apparatus that can prevent or greatly reduce the spillage of oil from the cargo tank of a ship when the tank is breached that reduces the risk of transporting oil cargo and reduces the cost compared to the incorporation of double hulls in ships.